The use of diagnostic tests in the clinical testing of patients has become increasingly common in recent years. For example the quantitative determination of lactate dehydrogenase (LDH) is extremely important in the detection of heart diseases. Following myocardial infarctions, the level of LDH in the blood rises noticeably over its normal concentration. The early detection of abnormal levels of LDH can therefore lead to a more accurate and rapid diagnosis of heart maladies.
Because early diagnosis of abnormal heart conditions is so important, a test for the detection of variation in LDH in the blood must be rapid and simple. It also must be highly accurate over a broad range of LDH concentrations encountered in patient testing.
A significant contribution in the field of clinical chemistry was the development of dry-to-the-touch multilayer analytical elements which could be used for simple, rapid and highly accurate testing of biological fluids. Such elements are described, for example, in U.S. Pat. Nos. 3,992,158 (issued Nov. 16, 1976 Przybylowicz et al) and 4,258,001 (issued Mar. 24, 1981 to Pierce et al). The Pierce et al reference, for example, describes multilayer elements comprising particulate spreading layers. These layers can also contain one or more surfactants to improve layer wettability.
European Patent Application 83/902727 (published Mar. 1, 1984 corresponding to PCT 84/00779) describes an assay for LDH carried out with a multilayer analytical element having a porous spreading layer containing a binder and microcrystalline cellulose (commercially available as Avicel.TM.). However, such elements containing microcrystalline cellulose spreading layers are difficult to manufacture on a large scale. Further, the assay described in this reference requires a blank subtraction step to reduce error. Hence, alternative materials were sought for making elements useful for LDH assays.
It was found that elements for LDH determination containing the beaded spreading layers described in the Pierce et al reference noted above are much easier to manufacture on a large scale than the Avicel.TM.-containing elements. It was also found that the rate curves obtained in assays with these elements are more linear than the Avicel.TM.-containing elements, even if no blank subtraction is made. However, it was observed that the assay of LDH with the Pierce et al elements was uunacceptably imprecise, especially at the lower levels of LDH. Blank subtraction does not eliminate this problem.
It would be highly desirable to have an element for LDH assay which is easily manufactured on a large scale, and which also exhibits acceptable precision over the entire range of LDH generally encountered in patient testing. It would also be desirable to avoid blank subtraction because it complicates the assay and requires additional equipment and computer software in automated analyzers.